1. Field of the Invention
This invention is directed to a system for resisting and preventing collapse of mine roofs and adjacent sides, and more particularly to multiple cable rock anchors which can be bent to permit longer cable lengths to be installed in low ceiling passages, and which cables can be rotated into position to penetrate and mix the resinous anchor materials.
2. Description of the Prior Art
Several types of anchor bolt systems have been used in the past to provide the support of mine roofs, and to provide long term stability to the system. Typical anchor bolts are seen in U.S. Pat. Nos. 3,226,934 of Jan. 4, 1966, 4,303,354 of Dec. 1, 1981 , 4,378,180 of Mar. 29, 1982 and 4,518,292 of May 21, 1985. Other types of anchor bolts are seen in U.S. Pat. Nos. 4,369,003 of Jan. 18, 1983 and 4,477,209 of Oct. 16, 1984. Rock anchor systems employing cables are shown in U.S. Pat. Nos. 3,913,338 of Oct. 21, 1975 and 4,160,615 of Jul. 10, 1979. Long cables have been used in boreholes to allow placement of anchor cables in the geological material to be mined. These systems essentially presupport the geologic material before mining takes place. In some systems, a cable or cables, with an attached air breather tube, are placed in the borehole to its full depth. The collar of the borehole is sealed around the cables, air breather tube, and a short insertion tube. Cementeous material is pumped through the insertion tube until it flows out of the air breather tube, assuring complete filling of the annulus area around the cable or cables. Cables are not rotated as no mixing of cementeous material is needed. Over a period of time, the cement sets up and the anchored cables serve to stabilize the rock mass. The above described system has been used throughout the world in mining for the past twenty years to stabilize geologic material in and around large excavations underground.
In more recent times, development and use of shorter cable bolts anchored in resinous materials has been employed by the mining industry. Research and development has proceeded rapidly due to the need for longer length rock anchors in the coal industry, particularly to stabilize the rock masses around headgate and tailgate entries associated with longwall mining. Coupled rod rock anchor systems have been used but their cost is high, installation is slow and large diameter boreholes must be used to allow passage of the couplings. For this reason, mine operators are desirous of using cable rock anchors which can be bent to allow longer than seam height, high strength rock anchors to be installed efficiently.
In the soft rock mining industry, coal, salt, potash, etc., it is common practice to rotary drill small diameter boreholes for rock anchors. Coupled large rod high strength anchors require that large diameter boreholes be drilled to provide room for the couplings, rods, and resinous material. Cables are normally four times as strong per unit weight as comparable strength rods. Thus, small boreholes can be used to obtain the same anchor strength. For example, a 0.6 inch diameter, 7-strand cable, in a one inch diameter borehole, resin anchored, is comparable to a one inch diameter solid coupled rod anchor in a 15/8 inch diameter borehole resin anchored.
Scott, in his U.S. Pat. No. 5,253,960, described a cable bolt for the foregoing application. A single cable is anchored in resinous material and can be quickly placed in lengths longer than seam height. Cable sizes most often used in industry are 7-strand solid wire cables of nominal 0.5 or 0.6 inches in diameter. These cables have near 20 and 30 ton load carrying capacity respectively. They all can be bent manually by a man to a near 4 to 5 foot diameter circle, and this capability allows them to be shipped in coils and manually bent in low seams, say 4 to 5 feet, for insertion into long boreholes. Large diameter individual cables are too stiff for manual handling and have not been used by industry.
To further increase the anchorage, it is desirable to place more than one cable in a single borehole and be able to install it in as small a borehole as possible, and to also install it with existing equipment available in the mining industry. This invention allows the joining of several cables in a bore, wherein the cables are the same or have differing diameters and strengths, using a single unifying attachment device or nut to allow rotation of all cables simultaneously to embed the cables with each other in resinous or cementeous material to provide a higher strength rock anchor than was previously possible. Cables are twisted during insertion to stiffen the system and provide a superior mixing system when rotated into resinous materials.